1. Field of the Invention
The present invention relates to fractionation of air by selective adsorption and is particularly concerned with controlling a vacuum swing system that recovers from air separate product fractions including a high purity nitrogen product and an oxygen-enriched gas fraction.
2. Background Art
Many systems are described in the prior art for automatically controlling various steps in the separation of gases by preferential adsorption. Typical among these are the following patents that generally disclose the use of selected gas separation process parameters for controlling the cyclic operation of predetermined process steps.
The control system disclosed in U.S. Pat. No. 3,703,068 monitors the pressure in the adsorbent beds during repressurization and terminates repressurization of the beds when a predetermined pressure level is obtained. The control system also initiates the appropriate cycle changes in other sorbent beds. Control of gas adsorption is obtained by altering the flow rate of the gas in order to provide the proper volume of feed gas that the sorbent bed is capable of handling during the adsorption step. U.S. Pat. No. 4,047,904 shows switching of the adsorption step to the desorption step when the control system detects that the composition of the exhaust gas at the effluent end of the sorbent bed exceeds a predetermined level. The flow rate of the gas remains constant. In U.S. Pat. No. 4,247,311, a microprocessor is used to monitor the adsorption column pressure during regeneration for the purpose of controlling the amount of gas used during the purge step. Disclosed in U.S. Pat. No. 4,194,890 is a pressure swing adsorption process where the composition of the effluent gas from one of the first adsorption beds is monitored to determine when adsorption has ended in that bed. The control system terminates the adsorption step based on the percent composition of the effluent gas from the adsorption column and initiates the next step of the process. U.S. Pat. No. 4,168,149 discloses a control system that senses the level of a vacuum in an adsorption bed that is being regenerated and switches the process to the next step when a predetermined level of vacuum is reached.
In the present invention, there is provided automatic control of the air fractionation system disclosed in U.S. Pat. No. 4,013,429 which is incorporated herein by reference. This patent discloses a vacuum swing adsorption process comprising the following steps:
(1) an adsorption step during which atmospheric air is introduced into at least one train of beds comprising in sequence, a pretreatment bed and an air fractionation bed that provides selective adsorption for separate recovery of high purity nitrogen and oxygen-enriched product gas;
(2) a rinse step during which the train of beds becomes saturated with the high purity nitrogen;
(3) an evacuation step during which nitrogen-enriched gas is withdrawn from the train of beds;
(4) a pressurization step during which the pressure in the train of beds is raised to near ambient pressure level; and
(5) the sequence is repeated starting with step one.
The above described prior art is limited in that it does not show how to control the operation of such a vacuum swing adsorption system based on the use of both enriched product gas composition and column pressure while maintaining flow rate substantially constant and varying cycle time.
Accordingly, an object of the present invention is a control system and method for a vacuum swing adsorption system that detects the percent concentration of a product gas and column pressure at predetermined steps of the process for the purpose of controlling each step of the process.
Another object of this invention is to keep gas flow rate substantially constant while controlling the time duration of the steps of adsorption and rinse.